# wire length, clarification

• Subject: wire length, clarification
• From: I am the NRA <pierson-at-cimcad.enet.dec-dot-com>
• Date: Mon, 28 Nov 94 21:22:06 EST
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```>>	Given a bunch of 22 ga. magnet wire and a 4" thin wall PVC pipe for
>>	a form, design a coil that works well.  I plan on figuring out
>>	if the wire diameter will allow between 400 and 1000 turns and
>>	produce a coil with a height to width ratio of about 4:1.
>>	Then, calculate this length, and figure out the 1/4 wave frequency
>>	based on this length.
>	How?  Its NOT the same as the length of the wire.  The wire length will
>	turn out to be _roughly_ twice the 1/4 wave, for most coils.  That is,
>	100 meters of wire will resonate at 50 meters.  ROUGHLY.  Ther is cut
>	and try here.

On rereading, i find myself unclear in the above.  Any frequency has an
associated wave length.  Roughly, a single isolated conductor
(1/4 wave over gorund, in radio talk...) will resonate at the point
where it is 1/4 wave long.  That is: a one meter wire will be resonant
at 4 meters.  (call it 70 MHz, roughly...).  But.  Take that same wire
and wind it onto a single layer coil.  It Now "looks" "longer".  It will
now resonate at (roughly) 2 meters (call it 144 MHz...8)>>).  The exact
frequency will depend on the length to width ratio, turn spacing, etc.
For tall skinny coils, it tends to 2-1.  For shorter fatter coils it
tends back towards to 1-1 but does not get there.  The math is not
exact.

(The 1/4 wave wire will ALSO resonate at 144, in a different mode,
and 288, and......)

regards
dwp

```